This invention is directed to a method for securing wire leads to a high temperature ceramic sensor without injuring the sensor or substantially affecting its performance. This invention is also directed to the lead attachment structure which results from the method.
An article entitled "High Temperature Metal Ceramic Seals" in Ceramic Age, Vol. 63-64 No. 4, April, 1954, states that three processes have been developed for bonding metal to ceramic. The so-called "Telefunken" process consists of bonding molybdenum powder to the ceramic and then brazing with silver. A second hydride method uses a layer of TiH.sub.2 or ZrH.sub.2 on the ceramic followed by a silver, copper or lead braze. The bond in this method is formed when the hydride is thermally decomposed. In the third method, a bond is effected by melting on the ceramic an alloy of zirconium or titanium and a brazing metal such as silver or copper.
Japanese Application No. 56-119959 published Feb. 7, 1983 entitled "Production of Ceramic Sensor" describes the production of a ceramic sensor for measuring the density of oxygen in a gas. A paste of a high melting point metal such as platinum is applied to the ceramic sensor and forms electrodes thereon.
In U.S. Pat. No. 3,339,267 issued Sept. 5, 1967 to R. L. Bronnes et al. entitled "Metallizing Non-Metals", the preferred process consists of cathodic sputtering a layer of tantalum onto the ceramic, and then sputtering a layer of platinum over the tantalum layer. A wire lead is soldered to the platinum to establish connection with the ceramic material.
An examined Japanese document No. 47-28590 published in 1972 is directed to joining metallic bodies plated with copper to a ceramic body by metallizing the surface of the ceramic with nickel or a nickel alloy and heating it 780-850 degrees C. in a non-oxidizing atmosphere.
Although various ways have been used to attach wire leads to ceramic sensors, serious problems arise when the attachment structure and sensor must function at high temperatures, i.e., approaching 600 degrees C. A good bond (adhesion) of the lead to the sensor is necessary. Also, the attachment method should not significantly change the response characteristic of the sensor nor degrade the ceramic composition. Where the electrical resistance of the sensor varies in response to the sensed parameter, such as temperature or humidity, the resistance of the lead connection structure should be low relative to the resistance range of the sensor. Establishing such a connection to a ceramic sensor which may be subjected to high temperatures presents problems which are addressed by the present invention.